Often it is desirable to provide a plurality of directional predefined radiation patterns, or antenna beams, associated with an antenna structure of a wireless communication network. For example, in cellular telecommunications, including PCS systems, multiple substantially non-overlapping antenna beams are often utilized to provide communication throughout the area of a cell.
The multiple antenna beams of a communication system may be generated through use of a planar or cylindrical array of antenna elements, for example, where a signal is provided to the individual antenna elements having a predetermined phase relationship (i.e., a phased array). This phase relationship causes the signal simulcast from the various antenna elements of the array to destructively and beneficially combine to form the desired radiation pattern. There are a number of methods of beam forming using matrix type beam forming networks, such as Butler matrixes.
Controlling interference experienced in wireless communication, such as may be caused by multiple users of a particular service and/or various radiating structures of a service or different services providing communication coverage within the same or different geographical areas, is a concern. Moreover, as the use of wireless communications increases, such as through the deployment of new services and/or the increased utilization of existing services, the need for interference reduction schemes becomes more pronounced.
For example, in code division multiple access (CDMA) networks a number of communication signals, each associated with a different user or communication unit, operate over the same frequency band simultaneously. Each communication unit is assigned a distinct, pseudo-random, chip code which identifies signals associated with the communication unit. The communication units use this chip code to pseudo-randomly spread their transmitted signal over the allotted frequency band. Accordingly, signals may be communicated from each such unit over the same frequency band and a receiver may despread a desired signal associated with a particular communication unit.
However, despreading of the desired communication unit's signal results in the receiver not only receiving the energy of this desired signal, but also a portion of the energies of other communication units operating over the same frequency band. Accordingly, CDMA networks are interference limited, i.e., the number of communication units using the same frequency band, while maintaining an acceptable signal quality, is determined by the total energy level within the frequency band at the receiver. Therefore, it is desirable to limit reception of unnecessary energy at any of the network's communication devices.
In the past, interference reduction in some wireless communication systems, such as the aforementioned CDMA cellular systems, has been accomplished to an extent through physically adjusting the antenna array to limit radiation of signals to within a predefined area. Accordingly, areas of influence of neighboring communication arrays may be defined which are appreciably smaller than the array is capable of communicating in. As such, radiation and reception of signals is restricted to substantially only the area of a predefined, substantially non-overlapping, cell.
Changes in the environment surrounding a communication array or changes at a neighboring communication array may require adjustment of the radiation pattern of a particular communication array. Specifically, seasonal changes around a base transceiver station (BTS) site can cause changes in propagation losses of the signal radiated from a BTS. For example, during fall and winter deciduous foliage loss can cause a decrease in signal path loss. This can result in unintentional interference into neighboring BTS operating areas or cells as the radiation pattern of the affected BTS will effectively enlarge due to the reduced propagation losses.
Likewise, an anomaly affecting a neighboring BTS may cause an increase in signal path loss, or complete interruption in the signal, therefore necessitating the expansion of the radiation patterns associated with various neighboring BTSes in order to provide coverage in the affected areas.
Previously, crews have had to be dispatched to purposely tilt BTS antennas up or down to minimize interference or provide coverage in neighboring areas. Likewise, crews have again had to be dispatched when the anomaly affecting the signal has dissipated or been resolved. Such adjustment is typically accomplished in concert with observation of field measurement, such as may be available from drive testing or by the results of operation statistical records. It becomes readily apparent that compensation for such anomalies, even occurring only seasonally, can be quite expensive. Furthermore, as the communication system grows in complexity, more such adjustments have to be made to bring the system back up to full operating capacity.
Furthermore, physical adjustment of an antenna array, including the multiple beam forming arrays discussed above, suffers from additional undesired effects. For example, because the beams of such a multiple beam array are steered away from the broadside, physical down-tilt of a panel will not result in the same size radiation pattern for each of the multiple beams. Specifically, the antenna beams having a less acute angle from the broadside will result in a smaller radiation pattern as experienced on the surface than will the antenna beams having a more acute angle from the broadside.
Additionally, physical adjustment of the antenna array which produces the above mentioned multiple antenna beams necessarily results in adjustment of every one of the multiple beams. However, it may be desirable to independently adjust the beams. For example, the aforementioned anomaly affecting radiation of signals may affect only certain antenna beams of an array and, therefore, only a subset of the antenna beams require adjustment. Likewise, adjustment of only a selected antenna beam in order to provide communication to a particular mobile communication unit may be desirable. However, current systems do not provide for the adjustment of individual antenna beams of an antenna array.
Additionally, to improve communications it is often desirable to provide for higher gain at the antennas. A high gain antenna may provide a usable signal, where a lesser gain antenna may not, through such advantages as an improved signal to noise ratio for a desired signal. However, typically higher gain, such as with a planar panel antenna, results in a larger aperture area. Such a larger aperture, however, is often undesirable due to higher wind loading (higher air resistance). Moreover, larger aperture antennas are often unsuited for use in, for example, metropolitan areas where site aesthetics zoning are often of great concern.
Further control of interference and improvement in communications may be had through antenna beam side lobe control. Through side lobe control, substantially only desired areas may be included in the antenna beam, thus avoiding energy radiated from undesired directions in the receive link and radiating energy in undesired directions in the transmit link. However, often in the past antenna beam side lobe control has been accomplished through the removal of antenna elements in outer columns of the phased array. However, this solution results in a reduction in antenna aperture, and thus gain, as well as an undesirable power balance, i.e., the remaining elements are energized with more energy than the inner column elements if no attempt is made to reduce the total power to the outer columns.
If the power is not properly balanced among the antenna columns, such as providing the same excitation energy to each column of the array including the antenna columns having a reduced number of antenna elements, side lobe levels will increase. This is because the energy of with each antenna column will be divided among the antenna elements associated with the column. Where there are fewer antenna elements, each element will be provided more energy as compared to antenna columns having more antenna elements. Accordingly, providing a signal of equal power to each antenna column of a prior art array adapted for side lobe level control typically results in energization of the elements in an aperture distribution approaching an inverse cosine distribution. As will be appreciated by one of skill in the art, such aperture distribution applied to a typical prior art planar array produces substantial side lobe levels.
Prior art attempts to balance the power among such antenna columns introduce additional problems. For example, antenna feed systems which are adapted to compensate for the removal of the antenna elements are very complex. Attempting to compensate for the excess energy provided to the antenna columns having fewer antenna elements through such means as resistive loads to dissipate the energy introduce such problems as causing intermodulation products etc.
Accordingly, a need exists in the art for a system and method providing elevation "down-tilt" of an antenna array providing illumination of a predetermined area in order to reduce interference and allow frequency reuse by additional such antenna systems.
A further need exists in the art for a system and method allowing for simplified adjustment of elevation down-tilt of an antenna array.
A still further need exists in the art for a system and method which provide for automated elevation control of the various beams comprising a radiation pattern.
A yet further need exists in the art for a system and method providing elevation control of the various antenna beams of an antenna array on a per beam basis.
A further need exists in the art for a system providing improved antenna gain without resulting in undesired wind loading.
A still further need exists in the art for a system providing antenna beam side lobe control without substantially compromising antenna aperture. Additionally, a need exists in the art for antenna beam side lobe control which does not introduce problems with respect to power balancing between antenna elements of phased antenna columns.